Multicellular organisms produce a battery of antimicrobial peptides and proteins to defend themselves against microbial attack or injury. Many of these induced peptides and proteins possess broad antimicrobial activity against Gram-positive and/or Gram-negative bacteria (Boman, H. G. (1995) Annu. Rev. Immunol. 13:61–92). This defense system, called “innate immunity,” may represent a chemical barrier that organisms deploy to stop dangerous microbes at their point of contact.
The peptides and proteins produced in response to microbial attack tend to work very differently from conventional antibiotics. Antibiotics work to block a crucial protein in an invading microbe. The mode of action of the antimicrobial defensive proteins varies. In some instances, they punch holes in a microbe's membranes and disrupt internal signaling of the microbe. In other instances, they may act to increase the host cell immune activity.
Several antimicrobial peptides have been isolated and their structures partially characterized. The defensins, one type of the antimicrobial peptides, are cysteine-rich peptides. Defensins have been isolated from insects and mammals. Insect defensins are 34–43 amino acid peptides with three disulfide bridges. They are produced by the insect fat body (Hoffmann et al. (1992) Immunol. Today 13:411–15). They have been shown to disrupt the permeability of the cytoplasmic membrane of Micrococcus luteus, resulting from the formation of voltage-dependent ion channels in the cytoplasmic membrane (Cociancich et al. (1993) J. Biol. Chem. 268:19239–19245).
Thionins are another group of small cysteine-rich antimicrobial peptides. Thionins are thought to play a role in the protection of plants against microbial infection. They are found in the seed endosperm, stems, roots, and in etiolated or pathogen stressed leaves of many plant species (Bohlmann et al. (1991) Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:227–240). Thionins display toxicity to bacteria, fungi, yeasts, and even various mammalian cell types.
Disease in plants has many causes including fungi, viruses, bacteria, and nematodes. Phytopathogenic fungi have resulted in significant annual crop yield losses as well as devastating epidemics. Additionally, plant disease outbreaks have resulted in catastrophic crop failures that have triggered famines and caused major social change.
Molecular methods of crop protection not only have the potential to implement novel mechanisms for disease resistance, but can also be implemented more quickly than traditional breeding methods. Accordingly, molecular methods are needed to supplement traditional breeding methods to protect plants from pathogen attack.
Plant pathogenic fungi attack all of the approximately 300,000 species of flowering plants, but a single plant species can be host to only a few fungal species, and most fungi usually have a limited host range. It is for this reason that the best general strategy to date for controlling plant fungal disease has been to use resistant cultivars selected or developed by plant breeders. Unfortunately, even with the use of resistant cultivars, the potential for serious crop disease epidemics persists today, as evidenced by outbreaks of Victoria oat and southern corn leaf blight.
Accordingly, molecular methods utilizing the resistance mechanisms of naturally occurring plant insect pests to enhance plant disease resistance to microbes, particularly pathogenic fungi, are desirable.